https://www.billingsclinic.com/foundation/3d-printed-surgical-mask/
In fact, if you own injection molding equipment, adapt this design and start to pump them out by the thousands.
CPAP masks are a good example of what you can do to get a decent seal with a hard plastic, but they take a lot of trial and error to get right, and there are hundreds of different models on the market.
You have to address the sealing issue first. If you don’t, then nothing else really matters — unless you’re just making vibrantly colored cloth masks to help encourage you to stop touching your face and to scare others into staying far away from you.
Yes, you are right, getting a good seal with a hard plastic is difficult (unless you impale it into your skin). I wonder how carefully they adjust their N95's to have a good seal? If you don't, particularly at the nose bridge, you can have some pretty big gaps.